The flatness of a surface as it is heated and cooled is desired to be known across the distribution chain of microelectronics design and manufacturing companies. Flatness over temperature, or warpage, is most commonly measured for surface mount components that are subject to a reflow temperature profile. The reflow temperature profile is used in microelectronics assembly process in which samples are heated to a temperature at which metals in numerous areas in the surface mount components liquefy at the peaks of the reflow profile, coalesce together, and refreeze during cooling. The now connected/formed metal joints serve as multiple electrical connections within an electronic sample. The term surface mount technology is generally used to describe this type of reflow attachment where one or more surface mount components are attached to an underlying board. Certain levels of warpage in surface mount samples can cause problems in creating the electrical connections between sample and board.
Fast and full field warpage data sets are required to meet industry demands for measuring warpage in microelectronics samples, as simulating the temperatures and timing of the reflow profile requires gathering large quantities of surface flatness data in a short amount of time (1-4 seconds). Some original efforts to meet these demands were claimed in U.S. Pat. No. 5,601,364 to Ume (1997), describing an apparatus for measuring thermally induced warpage in printed circuit boards (PCBs). This apparatus used heating elements and a shadow moiré technique to look at flatness. Accuracy of this described apparatus was viable for overall flatness measurement of PCBs in this era. However, the heating methods and application of a shadow moiré technique described in the Ume patent neither covered the thermal demands nor warpage measurement accuracy demands that would become a requirement of surface mount components in the coming years. The Ume patent is founded based on multiple thermal heating concepts as well as shadow moiré concepts. A shadow moiré system is comprised of a grating, a camera, and a directional light source. A grating is a piece of glass with alternating opaque lines and transparent lines spaced in a constant periodic pattern.
In order to meet accuracy demands needed for the emerging surface mount technology market the concept of phase stepping is introduced into shadow moiré technology in U.S. Pat. No. 5,969,819 to Wang (1999). The concept of phase stepping is still relevant to the shadow moiré system herein. However, developments in a flatness acquisition method are not part of the patentable art claims.
Other technologies have been pursued using an oven and a projection moiré system. Projection moiré is a similar technology to shadow moiré, using some similar mathematical approaches, camera images, and phase shifting to capture surface flatness. The two measurement techniques have different pros and cons in their use for capturing thermal warpage. At the time of this writing, shadow moiré remains the more popular technique for thermal warpage and the specific technique used in listed patentable claims. Shadow moiré has an advantage in thermal warpage measurement accuracy that is desirable for this technology. In particular shadow moiré has an advantage over fringe projection in scaling to larger measurement areas. Larger areas for measurement are needed, not only for larger samples, but also to increase the quantity of samples that can be run in a single thermal profile, improving equipment throughput.
Other approaches for heating have also been pursued in conjuncture with a shadow moiré technique. U.S. Pat. No. 9,383,300 to Chiavone and Gheesling (2016) describes a convection based solution for heating samples through reflow temperatures and measuring with a shadow moiré technique. This approach is valuable, but it also has certain limitations. Due to the need for a grating glass above and near to a sample under a shadow moiré test, heated air must blow in from the perimeter of a sample area that is to be heated. Also, restrictions apply to a shadow moiré metrology system due to the grating glass being disposed above and near to the sample. With heated air coming in from only the sides of the sample area, the approach of Chiavone and Gheesling has limitations with respect to the allowable size of said sample area. In practice, the patent is used in a 70 mm diameter circle. The usable size for a convection based shadow moiré system is restricted based on two requirements:                a) Air speed: Increasing velocity of air can cause instability in the sample location, in that a sample can “flutter”, shift, or even fly away. The need for increased speed of flow is for both heating rates as well as the following requirement (b).        b) Temperature uniformity: Blowing hotter air from the perimeter of a cooler sample area will always cause a decrease in air temperature as energy is lost to structures and samples on the perimeter of the sample area. This causes surface on the perimeter of an area to be hotter and surfaces further inside to be cooler.        
The combination of the above mentioned patents leaves a need for a solution in which temperature uniformity can be maintained over a large area (>a 70 mm diameter circle in practice) while being able to reliably use a shadow moiré technique.